A three-month-old baby died in its mother’s arms earlier this month. For hours the mother, Gana, gently shook and stroked her son Claudio, apparently trying to restore movement to his lolling head and limp arms. People who watched were moved to tears — unfazed by the fact that Gana and Claudio were “only” gorillas in Münster zoo, northern Germany.

It wasn’t just witnesses who were moved. A British woman who read about Gana’s loss online posted this comment: “From one bereaved mother to another — Gana, you are in my thoughts. My baby boy died last June and you wouldn’t wish it on any form of life.”

Some, to be fair, reacted differently. One newspaper writer asked bluntly whether we are “too quick to project human feelings onto animals”. However, Dr Bill Sellers, a primatologist at Manchester University, believes gorillas experience pain and loss in a similar way to humans, “but of course it’s extremely difficult to prove scientifically”.

As Einstein said: “Not everything that counts can be counted, and not everything that can be counted, counts.” Only a few years ago doctors did not give anaesthetics to tiny babies, believing they did not feel pain. By focusing narrowly on specifics — in this case, the emotional capacity of animals — scientists may fail to take account of what seems obvious and meaningful to the rest of us. The scientific experience of the world must seem a bit like watching a football match at night, with a single spotlight instead of floodlights.

Many of those who commented on Gana’s story online took a robustly anti-science line, asking angrily how “experts” could be so idiotic. “Have they not heard a cow calling for days when her calves are removed?” asked one. Others described how dogs and cats had become “depressed” by the death of their own kind — and indeed by the loss of human companions. These people would turn the sceptics’ question on its head: “Haven’t we been rather slow to recognise that animals have emotions?”

The question goes to the heart of our way of life. If animals have feelings, it is much harder to justify experimenting on them in laboratories, ogling them in zoos and farming them intensively — or, indeed, at all. The academics attempting to resolve this fall into two camps. Behaviourists accept only the results of tests, rejecting any unproven suggestion that animals think or feel or are even capable of emotion. Ethologists, on the other hand, are prepared to draw conclusions from studies and observation, anecdote and personal observation.

Ethologists, these days, are in the ascendant. One of the best known is Marc Bekoff, professor of biology at the University of Colorado and co-founder with the primatologist Jane Goodall of the group Ethologists for the Ethical Treatment of Animals.

Sceptical behaviourists often ask him, “How do you know dogs and elephants feel joy or jealousy or embarrassment?”

Bekoff replies: “One retort is to say: how do you know they don’t? Darwin said there was continuity in evolution, so the differences between species are differences in degree rather than differences in kind. They’re shades of grey.”

“If we feel jealousy, then dogs and wolves and elephants and chimpanzees feel jealousy. Animal emotions are not necessarily identical to ours but there’s no reason to think they should be. Their hearts and stomachs and brains also differ from ours, but this doesn’t stop us from saying they have hearts, stomachs and brains. There’s dog joy and chimpanzee joy and pig joy, and dog grief, chimpanzee grief and pig grief.”

Although many people would feel comfortable associating emotions with large, charismatic mammals, hard evidence increasingly suggests that other animals are similarly capable. The neurobiologist Erich Jarvis of Duke University, North Carolina, argues that evolution has created more than one way to generate complex behaviour; and that they are comparable.

Some birds have evolved cognitive abilities far more complex than those of many mammals. Dr Nathan Emery, a neuropsychologist at Cambridge University’s department of zoology, suggests that in their cognitive ability, corvids — the bird family that includes crows, ravens, rooks, jackdaws, jays and magpies — rival the great apes and might well be considered “feathered apes”.

Esther Woolfson, author of a new book, Corvus: A Life with Birds, has lived for years with a variety of these feathered apes. Woolfson doesn’t believe that her birds understand every word she says — the claim beloved of pet owners everywhere — but she does believe they have emotions. “I have seen — or believe that I have seen — in birds, impatience, frustration, anxiety in the urge to impart news, affection, fear, amusement (the last being a difficult one, I admit, to prove, merely on the basis of watching the look on a magpie’s face as its booby-trap was successful) and, particularly, joy.”

One bird, Spike, would balance an object — a pamphlet, a rubber glove, a matchbox — on top of a half-open cupboard door, then wait until it fell onto the head of the next person to open the cupboard.

Her birds also seemed to empathise: “To have a magpie, on seeing me weep, hover on top of the fridge, wings outstretched, tremble for a few moments then fly down to my knee to crouch, squeaking quietly, edging ever nearer until his body was close against mine, seemed to me at the time, (as it does now) an act of an unexpected tenderness that I can interpret only as empathy. There may be other explanations of their behaviour, but I can’t think what they might be.”

Bekoff agrees that we can no longer associate emotion only with the charismatic mammals: “The fact is that fish show fear. Rodents can empathise. This is hard science. With birds and mammals there is no doubt that they have a very rich ensemble of emotions.”

Satish Kumar, editor of Resurgence magazine, was for several years a Jain monk. The Jain respect for life is extreme: Kumar didn’t wash his hair for years in case there were fleas in it. He gave up being a monk eventually, for other reasons, but still believes that all living beings should be respected.

“We are animals. And we have a kind of empathy with the animal kingdom. They’re our kin. There is a slight difference between a cat and a dog and a chimp and a female human and a male one and a black human and a white one. These differences are very small: 98% of our DNA is the same as in other animals such as primates,” Kumar says.

“There used to be a time when people thought that animals had no soul, just as they thought that slaves or Africans or women had no soul. We realised a long time ago, as Jains, that animals have souls. They do feel pain and joy. Mostly they feel what we feel. Animals have empathy and intelligence. We have to be humble and accept that we are only one kind of animal and these are others.”

Jains divide the living world into several categories. “Living things like trees and vegetation have only one sense — touch. Then you have two senses, touch and taste, the animals that eat. Then there are animals with a third sense, smell. Fourth are the ones that have sight, too. And then hearing. Intelligence is limited in these cases because they get their information through fewer senses than us,” says Kumar. “But look at people who are not literate. Literacy is a relatively new thing. Before that we had only an oral culture. That does not mean that people lacked intelligence; just techniques.

“So even mosquitos have something. Even viruses and fungi have intelligence. Nature is full of intelligence. That intelligence manifests in different ways. A tree knows how to bear fruit.”

Many people will reject this as sentimental nonsense, but scientific evidence is increasingly providing support for such ideas. Dodder, the parasitic plant, appears to “choose” which host plants to parasitise on the basis of an initial evaluation of a potential host’s nutritional status. Transplanted shoots are more likely to coil on (“accept”) host plants of high nutritional status and grow away from (“reject”) hosts of poor quality. Crucially, this acceptance or rejection occurs before any food has been taken from the host. We do not yet understand how the parasite evaluates the host’s food value.

However, intelligence is not the same as emotion. Studies of intelligence and ability have been around for ever — a new one last week showed that elephants can do maths.

Evidence of emotional capacity, conceivably older in evolutionary terms than intelligence, has the greater potential to change the way we treat animals. You might put an animal into a circus if it did tricks, but if you knew that this upset the animal you would take it out again. (Unless you were a psychopath, many of whom have been shown to be cruel to animals as well as humans.)

To Bekoff, the great distinction between living beings is whether they have eyes: “The eyes tell it all.

If we can stand it, we should look into the fear-filled eyes of animals who suffer at our hands, in horrible conditions of captivity, in slaughterhouses and research labs, fur farms, zoos, rodeos and circuses. Dare to look into the sunken eyes of animals who are afraid or feeling all sorts of pain and then try to deny to yourself and to others that these individuals are feeling anything. I bet you can’t.”

Bekoff abandoned a promising career at medical school for this reason. “A very intelligent cat looked at me and asked, ‘Why me?’ I couldn’t find the words to tell him why or how badly I felt for torturing and killing him.”

Strict behaviourists might laugh at this, saying the animal’s expression was merely a physical response to particular stimuli. But if they are consistent they must say the same about human emotions, too.

Marian Stamp Dawkins, professor of animal behaviour at Oxford University, points out that even in humans it is difficult to measure emotion: “There are three ways: we can listen to what people say they feel; measure body temperature and heart rate and hormonal levels; and observe behaviour. Unfortunately, the three emotional systems do not necessarily correlate with each other. Sometimes, for example, strong subjective emotions occur with no obvious autonomic changes — as when someone experiences a rapid switch from excitement to fear on a roller coaster.”

Ultimately, the minds and feelings of individuals other than ourselves are private. “Access is limited because we can’t really get into the head or heart of another being — and that includes other people,” says Bekoff.

“I often imagine a dinner table conversation between a scientist and his or her child concerning research in which the nature of mother–infant bonds is studied by taking the infant away from their mother.

“Child: ‘What did you do today?’

“Parent: ‘Oh, I removed two baby chimpanzees from their mother to see how they reacted to this treatment.’

“Child: ‘Do you think the baby minded being taken from her mother?’

“Parent: ‘Well, I’m not sure — that’s why I did it.’

“Child: ‘But what do you think the baby’s fighting to get back to her mother and her writhing and screaming meant?’